[Ksummit-discuss] [TECH TOPIC] Mainlining PREEMPT_RT

[Steven Rostedt]

On Wed, 14 Oct 2015 00:19:41 +0200
"Luis R. Rodriguez" <mcgrof at suse.com> wrote:

> On Wed, Oct 14, 2015 at 12:40:10AM +0200, Rafael J. Wysocki wrote:
> > On Tuesday, October 13, 2015 12:42:30 PM Steven Rostedt wrote:
> > > 
> > > Now that it was announced in Dublin last week that mainlining
> > > PREEMPT_RT has funding, I would like to propose a tech topic to discuss
> > > what is needed to be done in the near future.
> > > 
> > > Thomas Gleixner and his minions will be doing the brunt of the work,
> > > but there's issues that need to be addressed with the rest of the
> > > kernel. The various topics would include:
> > > 
> > >  CPU hotplug
> > >  preempt_disable and local_irq_disable annotations
> > >  new timer_wheel implementation
> > >  simple waitqueues
> > >  trylock boosting
> > > 
> > > Several of the Real Time developers will be present at kernel summit. I
> > > think this would be a great opportunity to have them be able to sit
> > > down together and discuss the remaining issues.
> > 
> > Well, that's not limited to the RT developers even.
> > 
> > Good topic.
> 
> How much of this is the above topics Vs lack of good semantics in general, or
> code which needs love / good design? I ask as we have a few 'semantic'
> discussions, and I have a feeling there might be overlap.

Well, CPU hotplug needs a rewrite. Preempt disabling and irq disabling
needs some commentary on what they are actually protecting. The timer
wheel is showing its age. Simple waitqueues can help make the majority
of the cases less complex. But the trylock boosting is -rt only.

As always, when -rt goes into mainline, we try hard to make sure those
changes improve mainline. Thus, we change designs and clean up code as
-rt goes in. We've been doing that since 2004, and nobody took
notice ;-)  That is, people saw improvements and never knew those
improvements were solely to get -rt working better.

-- Steve